Liquid crystal displays (LCDs) are dominant in the flat panel market. Almost all LCDs are based on nematic field effects. As such, LCDs acts as electro-optical polarization controllers. The nematic liquid crystal configurations in an LCD are aligned basically planar, vertically or twisted. The configurations serve to alter the polarization state of the incoming light. Light is transmitted or reflected between polarizers, depending on the polarization state of the output light from the liquid crystal configuration. Polarizers are essential for operation of LCD devices.
Except for the very small minority of liquid crystal devices, which are not based on nematic field-effects, all LCDs require two polarizers for operation. Exceptions are the cholesteric display (ChLCD) and the polymer dispersed liquid crystal display (PDLC). In case of ChLCDs, light is reflected, transmitted or scattered depending on the alignment of their short pitch cholesteric liquid crystals. In the case of PDLCs, random pockets of liquid crystals are formed inside a polymer matrix. The alignment state of these liquid crystal pockets determines whether PDLCs scatter or transmit incoming light.
The problem with ChLCD is that it does not provide enough broadband to cover the entire visible spectral range. In the reflective state, only a certain wavelength region such as green light is reflected. The other wavelengths are transmitted resulting in colored and angular dependent reflection. ChLCDs also require high driving voltages and are mostly operated in a bistable on-off mode. In case of PDLCs, light scattering occurs at all wavelengths with small wavelength dependence. However, the LC-domains are formed by random polymerization of a liquid crystal mixture doped with photo-sensitive monomers.
The polymerization process forms a polymer network comprising monomer liquid crystal pockets (droplets). The size and position of the pockets are random. In the zero volts scattering state, scattering is due to the refractive index mismatch between polymer matrix and liquid crystal. When a high voltage is applied across the liquid crystal/polymer layer, positive dielectric anisotropic liquid crystal molecules align parallel to the applied electric field. If the ordinary refractive index of the liquid crystal matches the refractive index of the polymer matrix and the liquid crystals are properly aligned only small residual scattering will occur, resulting in a clear transmissive state with little residual haze. However, the voltage required to change the alignment of the liquid crystal pockets into a truly clear state is very high (˜50-100V). Moreover, since the size of the pockets or droplets of liquid crystal is random, the change between light-scattering and transmission is a very sluggish function of applied voltage, making multiplex driving impossible. Thus, PDLCs can only be used as single pixel devices and are therefore not suitable for display applications. These basic drawbacks prevent displaying adequate information contents and CMOS addressing of PDLCs. Moreover, the large driving voltage and rather thick PDLC-layers required for adequate light scattering further limits the applicability of PDLCs.